1. Do you have a current lease -- and what are the terms of the lease
2. How long has the tenant lived in this address?
Finally, have you read the provisions of Just Cause for Eviction or Measure EE. If not, you should at least familiarize yourself with it.
Here's a site dedicated to tenant rights. http://www.cjjc.org/resources/know-your-rights.
It provides a list of reasons for eviction, what it will take to evict, and when a landlord can/tcan't evict.